Air & Space Smithsonian – September 2019

AttheMcDonald

Observatoryin

Texas,a newdish

wasbuiltforVLBI

(verylongbaseline

interferometry).By

observingquasars

inthedistant

universe,geodesists

hopetomeasure

locationsonEarth’s

surfacedowntothe

millimeter.

University of Colorado at Boulder. “Of course, you

spend about 1,000 times more money.”

With millimeter precision, researchers can put

GPS receivers on opposite sides of a geological

fault like California’s San Andreas and watch

how they move relative to one another, perhaps

hinting at a future earthquake. Or they can put

receivers around volcanoes, such as Kilauea in

Hawaii, to track how the ground moves up and

down as magma shifts beneath it.

GPS isn’t perfect. The satellites drift a tiny bit in

their orbits, so errors can creep in, undetected, over

time. So geodesists need other techniques to pro-

vide an independent check. “The way to improve

the reference frame is to run these instruments

side by side,” says Larson.

The second-line method, called satellite laser

ranging, bounces a laser beam off a satellite and

measures how long it takes to return. This tech-

nique got its start at Goddard in 1964, just four

years after the laser was invented. NASA scientists

fired a laser at a satellite called Beacon Explorer-B,

which was covered in reflective surfaces, and a

telescope measured the beam’s return. Calculations

based on that measurement provided the distance

to Earth’s center of mass.

Today laser ranging is done off many different

kinds of satellites, including NASA’s ICESat-2

launched last year, and a shiny metal-and-glass

ball called LAGEOS-2, launched in 1992. NASA

operates eight satellite laser ranging stations around

the globe, but most of them are outdated. “We

buy parts on eBay sometimes to maintain them,”

says Merkowitz.

Running a satellite laser-ranging system also

takesa lotoftimeandattention.Operatorstypically

usea radarsystemmountedalongsidethelaser

todetectairplanesthatmightstrayintothearea

andautomaticallyshutdownthelaser.McGarry

rememberstheolddayswhenoperatorshadto

scan the sky with binoculars. “I could only do that

for about an hour at a time,” she says.

The third tool available to geodesists is very

long baseline interferometry, or VLBI. It uses

large radio telescopes to clock when radio signals

arrive on Earth from quasars, the energetic hearts

of massive galaxies billions of light-years away. By

noting the difference in arrival times at different

VLBI stations around the globe, scientists can

calculate small variations in the rotation rate of

theplanet—duetofactorssuchasthechanging

seasons,thetides,andweatherpatterns.

VLBIobservationscomeinspurts,according

toFrankLemoine,a geodesyexpertatGoddard.

Typically,uptoa dozenVLBIstationsaround

theglobewilltracka setofquasarsforupto 24

hours,onceortwicea week.Theseobservations

aresupplementedbyone-hourdailysessions.

Occasionally,longercampaignslastingweekshelp

THE COORDINATE SYSTEM IS ACCURATE

TO WITHIN A FEW CENTIMETERS RELATIVE

TO THE CENTER OF THE EARTH. BUT IN

AN INCREASINGLY WIRED WORLD, THAT

LEVEL OF PRECISION IS NO LONGER GOOD

ENOUGH. “YOU WANT THE DRIVERLESS

CARS TO STAY IN THEIR LANE.”

September 2019 AIR&SPACE 53

COURTESY UNIVERSITY OF TEXAS